The present invention is directed to processes for treating ink compositions. More specifically, the present invention is directed to processes for treating ink compositions suitable for ink jet printing to improve the jetting characteristics thereof. One embodiment of the present invention is directed to a process which comprises treating a dye with a zeolite, followed by admixing the treated dye with an aqueous liquid vehicle to form an ink composition. Another embodiment of the present invention is directed to a process which comprises admixing a dye and an aqueous liquid vehicle to form an ink composition, followed by treating the ink composition with a zeolite. Yet another embodiment of the present invention is directed to a process which comprises admixing a dye and an aqueous liquid vehicle to form an ink composition, preparing an ink container having an exit opening and a storage area and having a zeolite filter situated between the exit opening and the storage area, and incorporating the ink composition into the ink container. Still another embodiment of the present invention is directed to an ink container comprising (a) an exit opening; (b) a storage area; and (c) a zeolite filter situated between the exit opening and the storage area. Another embodiment of the present invention is directed to an ink composition prepared by the process which comprises treating a dye with a zeolite, followed by admixing the treated dye with an aqueous liquid vehicle to form an ink composition. Yet another embodiment of the present invention is directed to an ink composition prepared by the process which comprises admixing a dye and an aqueous liquid vehicle to form an ink composition, followed by treating the ink composition with a zeolite.
Ink jet printing systems generally are of two types continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The other type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known and are described in, for example, U.S. Pat. No. 4,601,777, U.S. Pat. 4,251,824, U.S. Pat. 4,410,899, U.S. Pat. No. 4,412,224, and U.S. Pat. No. 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
Ink compositions for ink jet printing and treatment processes thereof are known. For example, U.S. Pat. No. 5,026,425 (Hindagoila et al.) discloses inks for ink-jet printing containing Direct Black 168 dye which have improved water resistance when the sodium cations on the dye are replaced with cations selected from the group consisting of ammonium, polyfunctional, and volatile amine cations.
U.S. Pat. No. 4,810,292 (Palmer et al.) discloses an aqueous-based ink composition for use in ink-jet printers. The composition comprises (a) a vehicle of about 5 to 95% water and the balance at least one glycol ether, such as diethylene glycol; (b) a dye, such as Food Black 2, having at least one negatively charged functional group per molecule, present in an amount up to about 10% of the vehicle composition; and (c) a cationic compound selected from the group consisting of alkanol ammonium compounds and cationic amide compounds, present in an amount such that there is at least one molecule of cationic compound for at least one of the negatively charged functional groups on the dye, the pH of the ink composition being maintained in the acidic region below about 7.
U.S. Pat. No. 4,786,327 (Wenzel et al.) discloses a process for reducing the crusting propensity of dyes, complexed with sodium or other cations, used in ink-jet print heads and for improving other properties of such dyes. The process comprises replacing at least a portion of the cations on such dyes with preselected cations, such as those of the alkali metals lithium, potassium, ammonium, and amines. A two-step process may be used, in which the sodium cations first are at least partially replaced with hydrogen cations by passing an aqueous solution of the dye through the hydrogen form of a strong acid ion exchange resin and the hydrogen cations subsequently are at least partially replaced with the preselected cations by neutralizing the hydrogen-containing dye solution with a base which contains the preselected cation species. Alternately, a one-step process, comprising passing the aqueous solution of the dye through an ion-exchange resin pre-loaded with the preselected cations may be employed.
Zeolites are known materials which generally are hydrated aluminosilicates containing hydrogen, oxygen, aluminum, and silicon arranged in an interconnecting lattice structure. The oxide composition of zeolites vary, depending on their desired application, with typical components including SiO.sub.2, Al.sub.2 O.sub.3, CaO, MgO, TiO.sub.2, Na.sub.2 O, K.sub.2 O, Fe.sub.2 O.sub.3, MnO, and the like. Zeolites are available in a variety of particle sizes, typically ranging from about 40 microns to millimeters in diameter.
One difficulty frequently encountered with thermal ink jet printing processes is kogation. Kogation refers to the formation of a solid deposit on the surface of the thermal ink jet printhead heater surface, frequently caused by thermal breakdown of the ink as the liquid ink is heated and vaporized. Eventually, the deposits can build up to a sufficient extent that they begin to act as an insulator between the heater element and the liquid ink, resulting in poor printer performance; examples of specific printing impairment which may occur include an increase in transit time and a reduction in drop ejection velocity, resulting in poor drop placement on the recording substrate, and a reduction in drop volume, resulting in a loss of ink coverage and image quality. Kogation can be associated with the presence in the ink of excess divalent or monovalent cations. These cations can also affect the latency and recoverability of some ink jet inks. Latency is a measure of the period of time at a particular humidity level during which the flow of ink through a nozzle or jet can be stopped from jetting while it contains the ink, and then subsequently restarted without clogging of the nozzle; latency in general should be as high as possible to enable restarting of the ink jet printer after extended idle periods. Many commercially available dyes are supplied at purity levels wherein undesirable levels of ionic materials are present in the dye products. Thus, it may be desirable to purify dye solutions to a very high level of purity to render them suitable for use in thermal ink jet inks. Well known methods of dye treatment, such as reverse osmosis, tend to be very expensive and time consuming, and require special equipment.
Accordingly, while known ink compositions and ink fabrication methods are suitable for their intended purposes, a need remains for improved methods of preparing ink jet inks wherein the dyes therein are purified of excess ionic materials. In addition, there is a need for methods of purifying dye materials and ink compositions which are inexpensive, rapid, and do not require special equipment. Further, a need exists for processes for treating ink compositions for use in ink jet printing wherein the treated ink exhibits reduced kogation. Additionally, there is a need for processes for treating ink compositions for use in ink jet printing wherein the treated ink exhibits improved latency and recoverability. Also, a need remains for processes for treating ink compositions for use in ink jet printing wherein the treated ink exhibits improved shelf life and improved long-term jetting characteristics. There is also a need for methods of treating dye compositions and ink compositions to remove ionic materials wherein the method can be tailored selectively to remove specific ions from the dyes or inks. In addition, there is a need for processes for preparing ink compositions for use in ink jet printing wherein ionic impurities introduced into the ink subsequent to purification of the dye can be removed.